In the production of silicon compositions, transition metal catalysts have long been known to promote the hydrosilation reaction. Each transition metal-catalyzed hydrosilation reaction differs dramatically such that it is difficult to predict which transition metal will efficiently catalyze the hydrosilation reaction of a specific hydridosilyl reactant with a particular unsaturated reactant. For example, the effect of substituents on the silicon atom on adduct yield obtained in the platinum (Pt)-catalyzed reactions with 1-alkenes is in the following order of activity (R=Et):
Cl3SiH greater than Cl2RSiH greater than (RO)3SiH greater than (RO)2RSiH greater than R3SiH
The general trend for Pt-catalyzed hydrosilation reactions is that chlorosilanes are more reactive than alkoxysilanes (Comprehensive Handbook on Hydrosilylation; B. Marciniec, Ed.; Pergamon Press, New York, 1992; Ch.4; J. L. Speier Adv. Organomet. Chem. 1979, 17, 407; E. Lukevics Russ. Chem. Rev. 1977, 46, 197). However, if one evaluates a different transition metal or olefin, the above trend may be different. For example, in the hydrosilation reaction of heptene with rhodium (Rh), the above trend is the reversed. Due to the relative importance of the Pt-catalyzed hydrosilation reaction in commercial production of organofunctional silanes, a process that improved both the reactivity and selectivity of alkoxysilanes in the hydrosilation relative to that seen with chlorosilanes would be valuable.
A number of patents in the art have disclosed that various promoters can increase the rates and/or selectivities of hydrosilation reactions. In terms of chemical structures or properties, the various types of promoters differ dramatically, such that it is not possible to predict which chemical structures or properties are important for promotion, or even which hydrosilation reactions may be promoted, since promotion will also depend on the chemical structures and properties of each of the hydridosilyl reactant, the unsaturated reactant, and the hydrosilation catalyst. For example, the reaction of trichlorosilane with allyl chloride is promoted by weak amines such as phenothiazine (V. T. Chuang U.S. Pat. No. 3,925,434), while the reaction of methyldichlorosilane with allyl chloride requires a more basic tertiary amine such as tributylamine (Ger. Patent 1,156,073; C. Hu et al. Fenzi Cuihua, 1988, 2, 38-43; see Chem. Abstr. 1989, 111, 78085 m). Both of those reactions can be promoted with a second hydridosilane (U.S. Pat. No. 4,614,812) through a different promotion mechanism. Alkali metal carbonates or bicarbonates promote hydrosilations of allylic amines with hydridoalkoxysilanes (U. S. Pat. No. 4,481,364). Other hydrosilation reactions are promoted by phosphines, oxygen gas (D. L. Kleyer et al. U.S. Pat. No. 5,359,111), oxygen-containing organics including aldehydes, unsaturated ketones (R. Reitmeier et al. U.S. Pat. No. 5,663,400, H. M. Bank et al. U.S. Pat. No. 5,623,083), tertiary alcohols and silylated derivatives thereof, and propargylic alcohols and silylated derivatives thereof (H. M. Bank et al. U.S. Pat. No. 5,756,795), inorganic or organic salts including sodium alkoxides and compounds of tin and cobalt, and other organic compounds, including alcohols, diols, ethers and esters. Carboxylic acids, along with ketones, and esters thereof, appear to promote platinum-catalyzed hydrosilation reactions between hydridoalkoxysilanes and allylamine (U.S. Ser. No. 415,268). The use of acetic acid in promoting hydrosilations involving trimethoxysilane has been coincidental with the use of vinylcyclohexene oxide as the olefin, since acetic acid was discovered to be an impurity derived from early processes to make that epoxyolefin using peracetic acid (U.S. Pat. No. 2,687,406), as well as allyl glycidyl ether j. Am. Chem. Soc. 1959, 81, 3350).
Hydrosilation promotion effects are narrowly specific, and an effective promoter may work for a single hydrosilation reaction between a specific hydridosilane and a specific olefin. In addition to increasing reaction rates, yields, or selectivities, a promoter may act by preventing undesirable side reactions, which reduce yields/selectivities, such as undesired polymerization or formation of less desirable isomeric products. For example, added methanol is disclosed as being effective in reducing the undesired beta-isomer content in reaction products from platinum-catalyzed hydrosilations between trimethoxysilane and the epoxyolefins, i.e., vinylcyclohexene monoepoxide and allyl glycidyl ether (H. Takai et al. U.S. Pat. No. 4,966,981).
The use of amines in the hydrosilation of hydridosilane and acrylonitrile has been reported extensively, particularly tertiary amines in the presence of copper (Cu) (B. A. Bluestein U.S. Pat. No. 2,971,970, 1961; Z. V. Belyakova et al. translation from Zhurnal Obshchei Khimii 1964, 34, 1480-1484; A. Rajkumar et al. Organometallics 1989, 8, 549-550; H. M. Bank U.S. Pat. No. 5,283,348, and U.S. Pat. No. 5,103,033). U.S. Pat. No. 4,292,434 (T. Lindner et al.) describes the preparation of an amine-platinum catalyst and its use in the hydrosilation reaction. K. R. Mehta et al. in U.S. Pat. No. 5,191,103 reported the use of sterically hindered amines, phosphines or their equivalent salts in the presence of a platinum catalyst to promote the hydrosilation reaction.
In addition to promoting the hydrosilation reaction, amines have been reported to be inhibitors for the hydrosilation reaction. For example G. Janik et al. in U.S. Pat. No. 4,584,361 reported that amines inhibited polyorganosiloxane compositions at temperatures below 40xc2x0 C., but not at 135xc2x0 C. Also R. P. Eckberg et al. reported the use of tertiary amines in the presence of both Rh and Pt catalysts to inhibit epoxy-polymerization in the production of epoxysilicones.
The hydrosilation reactions of many olefins, particularly amino-functional olefins, are either too slow or do not occur. For those olefins that do undergo hydrosilation, formation of the undesired xcex2-isomer is a competing side reaction. The type of silane employed also impacts the rate of reaction. Typically, sluggish hydrosilation reactions result in an increase of the competing side-reactions, e.g., olefin isomerization or polymerization. Accordingly, a process which improves the reactivity and selectivity of the transition metal-catalyzed hydrosilation reactions of olefins continues to be a commercially desirable objective.
In accordance with the invention, a process is provided which comprises reacting (a) hydridoalkoxysilane with (b) olefin in the presence of (c) platinum catalyst and (d) a weakly nucleophilic amine of the formula NZ1Z2Z3, wherein Z1 is an aryl, alkaryl, or aralkyl group of C6 to C20 carbon atoms, or an organosilyl group of the formula SiR3, wherein R is alkyl of C1 to C20 or aryl of C6 to C10; Z2 is hydrogen, alkyl of C1 to C20, an aryl, alkaryl, or aralkyl group of C6 to C20, or SiR3, wherein R is as previously defined; Z3 is the same as Z1 or Z2; and optionally two of Z1, Z2and Z3taken together with the nitrogen atom form an aromatic heterocyclic ring. The process of the invention exhibits improved yields and selectivities with respect to the desired reaction products.
This invention provides a process for improving the yields and rates of the hydrosilation of alkoxyhydridosilane under relatively mild conditions using a weakly nucleophilic amine in the presence of a hydrosilation catalyst.
Weakly nucleophilic amines containing substituents capable of xcfx80-interation with the amine""s lone pair of electrons such as aromatic or silicon-substituents can be employed in the practice of this invention. Thus, weakly nucleophilic amine promoters possess the general formulae NZ1Z2Z3 wherein Z1 is an aryl, alkaryl, or aralkyl group of six to twenty carbon atoms, or an organosilyl group of the formula SiR3, wherein R is an alkyl of C1 to C20, preferably C1 to C4, or aryl of C6 to C10; Z2 is hydrogen, alkyl of C1 to C20, preferably C1 to C4, an aryl, alkaryl, or aralkyl group of C6 to C20, or SiR3 wherein R is as previously defined; and Z3 is the same as Z1 or Z2. Optionally, two of Z1, Z2 and Z3 taken together may form an aromatic heterocyclic ring including the nitrogen atom. Weakly nucleophilic amines include, but are not limited to, aniline, hexamethyldisilazane, phenothiazine, aminonaphthalene, benzylamine, pyridine and their corresponding derivatives. Aniline, benzylamine, and hexamethyldisilazane are the preferred amines with this invention, the choice depending on the hydridosilane and olefin reactants.
Promotable hydridosilanes in general can be represented by the formula RnX3-nSiH, wherein R is a branched or linear alkyl group of 1 to 18 carbon atoms, a cyclic alkyl group of four to eight carbon atoms or an aryl, alkaryl, or aralkyl group of six to twelve carbon atoms, optionally containing halogen, oxygen or nitrogen substituents with the proviso that such substituents do not interfere with either hydrosilation or promotion, and X is an alkoxy group, selected from -OR, wherein R is as defined above, and n is 0, 1, or 2. The hydridosilanes may be alkoxysilanes selected from the group of trimethoxysilane, triethoxysilane, tri-n-propoxysilane, and triisopropoxysilane. Trimethoxysilane and triethoxysilane are preferred. Other hydridoalkoxysilanes include alkylalkoxysilanes such as methyldimethoxysilane, methyldiethoxysilane, dimethylmethoxysilane, and dimethylethoxysilane.
The olefins which can be employed in accordance with the invention are aliphatically unsaturated molecules, which may have certain functional substituents thereon. The term xe2x80x9colefinsxe2x80x9d utilized herein is being used in its broadest sense and therefore shall be understood to include alkenes, vinyl group-containing and allyl group-containing compounds. Terminal alkenes can be advantageously employed, such as the 1-alkenes, including ethylene, propylene, butene, pentene, hexene, octene, hexadecene, octadecene, trivinylcyclohexene, and the 2-alkyl-1-alkenes, such as 2-methylpropene, 2-methylbutene, diisobutylene, as well as non-terminal alkenes such as tertiary amylene and 2-butene. The 1-alkenes are preferred. Other suitable olefins include the epoxy olefins, such as vinylcyclohexene monoxide, allyl glycidyl ether, and allylic olefins, including, but not limited to allyl esters, allyl polyethers, and allylic tertiary amines, as well as their methallyl derivatives. Other olefins include the amino olefins, such as N-allylaniline, N,N-dimethallylamine, and N-ethylmethallylamine. Vinyl group-containing compounds include the vinyl esters and ethers, vinylsilanes, acrylates and methacrylates.
Catalysts include those which contain platinum and that function as either homogeneous or heterogeneous hydrosilation catalysts. Typical catalysts include chloroplatinic acid and various solutions thereof, including solutions wherein the chloroplatinic acid has been chemically modified, chloroplatinate salts and their solutions, vinylsiloxane complexes containing platinum and solutions thereof (Karstedt catalyst), olefin and diolefin complexes of platinum and solutions thereof, and platinum deposited as the metal on various substrates, including carbon, alumina, silica, organically modified silicas, or base metals. Platinum complexes containing strongly bound ligands such as phosphines, acetylacetonate groups, or amines, may be promotable with the proviso that such ligands must not interfere with either the hydrosilation or the promotion. The catalyst should be used at a level of 0.5 to 100 ppm based on total charge, preferably 5 to 50 ppm, most preferably 5 to 15 ppm.
Promotion by amines is not subject to limitations regarding equipment, relative to size or type of material of construction. A wide variety of laboratory or commercial scale equipment currently capable of running hydrosilation reactions may be used. The hydrosilation process may be run in a batch, semi-batch or continuous mode.
Reaction conditions are also not narrowly critical with regard to temperature, pressure, or the absence or presence of inert solvents. Conditions currently in use for various hydrosilation reactions can be used for the promoted hydrosilations. It is possible that effective promotion will be accompanied by the added advantages of lowering reaction temperature, and catalyst concentration, or both. Preferred reaction conditions include a temperature from about ambient temperature up to about 150xc2x0 C. with 60 to 120xc2x0 C. being most preferred. Generally, the process is carried out at a pressure of about 0.2 to 2.0 atmospheres (0.02-0.2 MPa), with ambient pressure being preferred, but operation at higher or lower pressures may be performed to maintain higher or lower reaction temperatures dependent on the volatilities of the respective reactants.
The residence time within the reactor is not critical but should be sufficient to achieve a satisfactory degree of conversion to the hydrosilated product, i.e.,  greater than 80%, within acceptable limits given the volume of the equipment and the desired rate of production. Typical acceptable residence times are on the order of 0.5 to 4 hours.
Preferably the olefin should be present at a molar excess of 5-20%, though a stoichiometric equivalence or a molar excess of the silane may be used. The use of promoters of the instant invention can allow the use of lower molar excesses of olefins due to reduction of the competing olefin isomerization side-reaction.
The amine may be present at the start of the hydrosilation, or may be added during the reaction if it is not proceeding well(Cautionxe2x80x94amine should not be added to incomplete reactions wherein significant quantities of both hydridosilyl reactant and olefinic reactant have accumulated, a rapid exothermic reaction may occur). The amines can be used at a concentration of 25 to 20,000 ppm (wt/wt); however, the preferred amine concentration is dependent on the olefin-silane system. The best mode of practice is to introduce the amine with the olefin and not with the alkoxysilane; although the amine can be introduced with the silane.
Promotion by amines is effective for those hydrosilation products which can be purified, as by distillation, and thusly separated from the amines, which may be lower or higher boilers which will be stripped or remain in the distillation residue, and can be isolated for disposal, or reused to promote a subsequent batch of product.